Rotational moulding is a process whereby a polymer (typically but not exclusively powdered polyethylene) is placed inside a female mould. The mould is normally in two or more parts and fully encloses the polymer. Typical mould materials are steel or aluminum with a steel or aluminum frame. This mould is then heated (normally by placing the mould in an oven however numerous other methods are used). As the mould is heated the polymer is distributed around the inside of the mould by rotating the mould through two perpendicular axes which themselves are perpendicular to gravity. As the mould heats up the polymer will tend to attach to the walls of the mould. The heat distribution around the inner surface of the mould is one of the major determinants of where the polymer will accumulate (the thickness of the final part will be determined principally by the heat distribution around the inner surfaces of the mould when it was made). Centrifical forces generally play a secondary role in determining polymer accumulation inside the mould. Once sufficient heating is provided the mould is cooled. Normally rotation is continued throughout cooling. After sufficient cooling is provided the rotation can stop and the mould can be opened and the formed part or parts extracted.
In addition to the polymer in the mould there is also often air or some other gas trapped inside the mould. Gases experience significant rates of thermal expansion in comparison to the mould materials. For this reason if the mould is sealed the pressure inside the mould will rise as the air in the mould is heated and will fall as the air inside the mould is cooled. In order to reduce the fluctuations in pressure it is common practice to place a ‘vent tube’ or breather tube into the mould. A vent tube is a tube that allows gas to flow from the outside of the mould to the inside of the mould. Often a wad of porous material such as cotton wool or fibre glass wool is placed into the tube to stop the polymer from falling out of the mould as the mould rotates. Often this packing material can produce a significant resistance to gas flow through the vent.
In practice we believe most moulds do not fully seal where the different parts of the mould come together (partlines). This results in the mould venting through the partline of the mould until such time as the partline become blocked with polymer. If gas flow through the vent is restricted then once the partlines become blocked the pressure will start to rise in the mould as the gas temperature inside the mould rises. Once cooling starts the pressure inside the mould will start to fall as the temperature of the gas inside the mould also falls. During cooling the gas inside the mould is sealed from the atmosphere surrounding the mould as the polymer normally completely coats the inside of the mould in its entirety. Assuming that the vent remains restricted a vacuum will be created within the moulded part. This vacuum can cause bubbles to be formed in the part in the area of the partline as gas attempts to enter the part within the mould to relieve the vacuum. As the polymer continues to harden with cooling and the vacuum persists then the part may be sucked away from the mould wall. This contributes towards deformation in the finished product.
A positive pressure added to the mould towards the end of the heating cycle is beneficial to production. Positive pressure can also be used during the cooling period to reduce warpage.
Vents can be used to act as one way valves. These valves could use an elastomer that opens to allow air to enter the mould and then close again to stop gas and powder from exiting the mould. Such one way valves would operate reliant on the pressure differential between the gas inside and the gas outside of the mould. Accordingly a positive pressure can be created inside the mould during the final stages of heating which ensures that vacuums are not experienced during the cooling phase.
The major short fall of such a valving system is that it is difficult to control the pressure that builds up within the mould. Moulders would be concerned that pressures within moulds do not appreciate to levels where moulds are damaged or people are injured.